Greiner successfully defended Ku-ring-gai in the 1981 state election, managing a modest swing in his favour even as the Liberal-National CountryCoalition was cut down to only 28 seats in total. Shortly after the election, he ran for the party leadership, but lost to John Dowd. However, in 1983, Greiner ousted Dowd in a leadership challenge. Highlighting allegations of corruption against the Australian Labor Party government of Premier Neville Wran during the 1984 election campaign, Greiner managed to cut the Labor government's previously overwhelming majority in half, from 41 seats to 21. In the process, he regained much of what the Coalition had lost in the previous two "Wranslides."

Wran retired in 1986, and was succeeded by Barrie Unsworth. In what proved to be a harbinger of things to come, the Liberals came within 54 votes of derailing Unsworth's bid to enter the lower house (he'd previously been a member of the Legislative Council), and managed to take Wran's old seat on a large swing. At the March 1988 election, he led the Coalition to a landslide victory. The Coalition scored a swing of over 8 percent and took 22 seats from Labor, including several Labor heartland seats where the Coalition had not come close to winning in years.

Greiner handed over a number of responsibilities previously associated with the Premier's Department to other ministers, but in a rare move, served as his own Treasurer, in order to focus on repairing the State's parlous financial position. The Greiner government, which promised "sensible, moderate but progressive" government, wasted no time in commencing its legislative agenda, announcing across-the-board spending cuts and plans to announce a mini-budget in June. A key government policy was to cut costs in education, including increasing charges for public education, eliminating free public transport for school students and reducing teaching staff (2,400 teaching staff and 800 support staff) through creating composite classes and closing smaller schools, while looking for public assets to sell (ultimately $340 million of assets were identified) and capital works projects which could be abandoned. However, it was forced to defer its plans to repeal Aboriginal land rights legislation (an election commitment) and reduce the power of the Ombudsman to investigate the police when Democrat and Call to Australia members of the Upper House combined with Labor to defeat these pieces of legislation.[3] By September 1988, having promised at the election to run the state like a business, the government were able to announce serious progress towards reducing the state debt and its first budget projected a surplus, and were trying to resolve housing pressures caused by rapidly increasing house prices (which rose from $65,000 to $165,000 in the twelve months to October).[4] Another election promise realised by the new Government was to create the Independent Commission Against Corruption (ICAC) capable of investigating allegations of corruption and graft against the government and within the state. In its first year, it investigated large donations to the Labor and National parties. Despite significant pressure from the parliamentary Nationals, whose leader was implicated in one of the investigations, Greiner refused to budge or to dilute ICAC's powers.[5]

By 1989, the government's position could be described as higher government charges and reduced government services. Increased taxes and fees on motor vehicle drivers, petrol, water, public transport and child care, as the aforementioned cuts to education took effect, the pay of health workers was frozen, workers' compensation to injured workers was limited (although this was later blocked by the Upper House) and state assets were sold off.[6] A series of strikes on the part of teachers and the growing unpopularity of Education Minister Terry Metherell caused problems for the Greiner government during the latter stages of its first term. In August 1989, the Industrial Commission ruled against the government in determining that class sizes were an industrial issue rather than the sole discretion of the Director-General. The government also cut rail services to the north coast, deeming them commercially unviable, and cut 8,000 State Rail Authority employees in an effort to offset the authority's $1 billion annual debt — however, this left the Pacific Highway as the only land transport link for several major towns, and in October and December 1989, two major crashes on the road, both involving passenger buses, claimed a combined total of 54 deaths and 55 injuries.[5] While Opposition Leader Bob Carr made the link between pressure on roads and withdrawal of the country services,[7] a coroner's report in April 1990 pinned the blame on the Federal Government and its "piecemeal" approach to repairs of the Pacific Highway. The government meanwhile reduced speed limits for heavy vehicles to 90 km/h after the accidents, but a work-to-rule campaign by the Transport Workers Union disrupted Sydney's peak-hour traffic and Greiner overrode the Transport Minister to restore the previous 100 km/h limit in late January 1990.[8]

In May 1990, Greiner asked Metherell to try and resolve the ongoing battle with the State's teachers, and they were offered a 9% pay rise, although the disputes continued. Lecturers in the TAFE system, also within Metherell's portfolio, joined them after the government indicated its willingness to implement a report by a private management consultant envisaging a public-private partnership and massive staffing cuts. Metherell resigned from his position in 1990 but the disputes with teachers continued.[8]

Regarded as a fiscal conservative, Greiner was still considered much further to the left than many of his fellow Liberals in terms of social policy. He criticised then Federal Opposition Leader John Howard's controversial comments on immigration policy during the late 1980s, and was widely respected within the ethnic community.

Buoyed by his government's strong performance in the polls, Greiner called a snap election for 25 May 1991. Despite widespread predictions by political and media commentators that Greiner would be easily re-elected to a second term, the impact of the Government's policies, particularly in terms of service cuts and increased charges, caused many voters to turn back to Labor. The election saw the Coalition win 52 percent of the two-party vote. However, much of the Coalition margin was wasted on landslide margins in its heartland, while Labor took back many marginal seats it had lost in its severe defeat of three years prior. The result was a hung Parliament, with the Coalition one seat short of a majority. Greiner was forced into a minority government, relying on support from four Independent MPs. His parliamentary majority was further eroded with the decision of Terry Metherell to become an Independent in late 1991, and with the loss of The Entrance in a 1992 by-election following a Court of Disputed Returns overthrowing the original result.

Greiner and Environment Minister Tim Moore decided to offer Liberal-turned-independent MP Terry Metherell an executive position in the Environmental Protection Authority. If Metherell accepted the position, he would have to resign his parliamentary seat, which the Liberal Party was confident of winning in a by-election. While Metherell initially agreed to the position on 10 April 1992, it was criticised by Labor and the independents, and documents were ultimately released showing he had applied for a job in the Premier's Department and then been seconded to the EPA, and had applied after the closing date, but was appointed within hours of his application. Greiner was accused of misleading the parliament, and in late April, Labor and the independents passed a no-confidence motion in Greiner's leadership (though, critically, not against the government) in the Legislative Assembly. The pressure led to Greiner moving that the Assembly refer the matter to ICAC. The inquiry began on 5 May, and following detailed evidence by Metherell that his resignation was part of a package negotiated with Greiner and Moore and the release of Metherell's diaries, Greiner and backbencher Brad Hazzard admitted their statements to the inquiry were wrong. Greiner as a witness could not recall 20 key events under investigation, and the inquiry heard that the director-general appointed Metherell when it was discovered he could not legally be appointed to the EPA.[9]

On 19 June, ICAC commissioner Ian Temby concluded that while Greiner had not acted criminally and had not set out to be corrupt, he would be seen "by a notional jury as conducting himself contrary to known and recognised standards of honesty and integrity".[10] Temby found strongest against Environment Minister Tim Moore, a friend of Metherell's who was central to the offer. However, the Commission did not recommend taking action against the two ministers, saying that this was properly the role of Parliament.[11] Greiner focused on the words "honesty and integrity" and argued he was only "technically corrupt", but by 21 June, it was clear the independents would support a vote of no confidence in Greiner and Moore. Greiner lodged a case with New South Wales Court of Appeal and argued any such motions would breach natural justice while the appeal was being heard, but Labor and the independents argued that the Parliament was the body which should decide Greiner's future. Labor tabled a no-confidence motion, which was due up for vote on 24 June.[9] The independents told Greiner that unless he resigned, they would withdraw their support from the government and support the no-confidence motion.[12] Accordingly, Greiner resigned, and was succeeded by John Fahey.[9]

Greiner successfully appealed against the finding in the New South Wales Court of Appeal, which in a 2-1 vote on 21 August 1992 overturned the ICAC findings.[13] The court found that ICAC had "exceeded its jurisdiction" in ruling against the two ministers[14] and granted "declaratory relief that the Commission's report was wrong in law".[15] Following the affair, a parliamentary committee inquiring into ICAC's powers in December 1992 recommended that Section 9 of the ICAC Act, on which the successful appeal was based, should be repealed as it was too narrow in defining corrupt conduct.[16] While the section was not repealed, a sub-section was ultimately added in 1994 which addressed the behaviour of ministers and members of parliament, and gave legislative enforcement to ministerial and parliamentary codes of conduct.[17]

He went on to hold directorships with many of Australia's leading companies. Greiner was chairman of the board of WD&HO Wills and then British American Tobacco Australia for the period 1996 to 2004.[19] In 2011, he was Chairman of Bradken, Citigroup Australia, The Nuance Group, QBE Lenders’ Mortgage Insurance, Blue Star Print Group and Playup; and Deputy Chairman of CHAMP Private Equity.[20] In May 2011, Greiner was appointed as Chairman of Infrastructure NSW by the O'Farrell government.[21]

In the few years after leaving State politics, Greiner had been touted for a tilt at Federal politics including the prospect of replacing Alexander Downer as Federal Liberal Leader and becoming Prime Minister by leading the Coalition to victory at the 1996 Federal election but none of this eventuated.

He is married to Kathryn GreinerAO, a former Councillor in the Sydney City Council, and daughter of Sir Bede Callaghan (a former Chairman of Commonwealth Bank and Chancellor of Newcastle University). They separated in 1995, then reconciled in 1996.[22] In October 2013 it was announced that they had again separated.[23] The couple have one son and one daughter.